1. Field of the Invention
The present invention relates to a liquid crystal display device of the simple matrix drive type, and particularly to a liquid crystal display device which inverts the polarity of the scanning voltage by applying positive and negative selecting voltages alternately to its scanning electrode group in order to achieve line-at-a-time scanning.
2. Description of the Prior Art
Conventionally, the so-called simple matrix driving of a liquid crystal cell, such as is provided with electrode groups arranged perpendicularly to each other on both sides of a liquid crystal layer, has been achieved by line-at-a-time scanning. Specifically, the electrodes of one electrode group is sequentially supplied with a high-level voltage, and, while the high-level voltage is being applied to one electrode, the other electrode group is supplied with voltages in accordance with the video signal. Meanwhile, to avoid applying a direct-current to the liquid crystal, it has been customary to invert the polarity of those voltages, as disclosed in Japanese Published Patent Application S57-57718.
Take for example a method which accomplishes alternating-current driving by providing an polarity-inverting signal that inverts its polarity frame by frame. According to this method, while the first frame is being scanned, the scanning electrodes are supplied with a voltage V0, and the signal electrodes for the picture elements to be displayed (selected picture elements) are supplied with a voltage V1 for the first frame; then, while the next frame is being scanned, the scanning electrodes are supplied with the voltage V1, and the signal electrodes are supplied with the voltage V0.
This method, however, results in increase of power consumption, because the liquid crystal causes a large capacitive load current to flow when the polarity-inverting signal switches its polarity. Moveover, the latest developments have made it possible to produce liquid crystal display devices with as many as 1,024 RGB.times.768 picture elements (color XGA, with 3,072 signal-side picture elements per line), as compared to conventionally common ones with 640.times.480 picture elements (VGA). Since these larger devices require accordingly higher speeds in data transfer and other processing, they necessitate integrated circuits that are capable of handling higher voltages at higher speeds. Attempts to realize such integrated circuits, however, have been unsuccessful to date, because, in integrated circuits, higher processing speeds usually conflict with their handling of higher voltages.
Trying to solve these problems, the applicant of the present application proposed, in U.S. patent application Ser. No. 08/553,868, a liquid crystal display device that satisfies the conflicting requirements as described above. In this display device, a large-value positive voltage, a large-value negative voltage and an intermediate voltage between the large-value positive and negative voltages are applied to the scanning electrode group, and two difference voltages close to the intermediate voltage are applied to the signal electrode group.
The large-value voltages are selecting voltages to select picture elements to be displayed, and the positive and negative voltages are alternately applied every scanning to invert the polarity. To the scanning electrodes other than the scanning electrode to which the selecting voltage is being applied is supplied with the intermediate voltage. Which one among the two difference voltages is applied to the signal electrodes is determined based on the polarity of the selecting voltage being applied and on the video signal. When the picture element at the intersection of a signal electrode and the scanning electrode to which the selecting voltage is being applied is to be displayed, the voltage which causes larger potential difference between the scanning and signal electrodes is supplied to that signal electrode.
The voltages applied in the above-mentioned liquid crystal display device are shown in FIGS. 8A and 8B. FIG. 8A shows output voltages of a scanning circuit which supplies voltages to the scanning electrodes and a signal circuit which supplies voltages to the signal electrodes, and FIG. 8A shows voltages applied to the liquid crystal. These figures show that the scanning voltage results from selecting either the positive or negative selecting voltage at constant time intervals; however, since which of the difference voltages outputted from the signal circuit is selected changes according as the video signal and the polarity-inverting signal change, the voltages in both cases are simultaneously shown in the figures, making the diagrams look like, as it were, beads in an abacus. This does not indicate, however, that both signal voltages may be selected at the same time just as shown in the figure, nor that the voltage waveforms change gradually.
This liquid crystal display device, however, has proved to cause ghosts as described in Japanese Published Utility Model Application H6-26890. Specifically, in a dot matrix display device with a large number of picture elements, when vertical or horizontal bars or boxes of fixed widths such as are found in a bar graph or the like are displayed, they are accompanied with dim shadow-like lines or bars appearing in their respective extension directions where picture elements are supposed to be turned off. This greatly degrades picture quality.
The liquid crystal display device has also proved to pose a new problem as follows. Some types of devices that are used in combination with the liquid crystal display device generate a display activating (DISP-OFF) signal, which serves as a display control signal, at approximately the same time as it outputs a frame (FLM) signal and a clock signal. When the liquid crystal display device in question is connected to a device that outputs the DISP-OFF signal earlier than usual as described above, the DISP-OFF signal, which is supplied to the scanning and signal circuits, may turn into an active state (H-level) prematurely, that is, before the bias voltage for driving the liquid crystal, which is generated by a DC--DC converter or other at the start-up of the display-on sequence, reaches a predetermined voltage. This usually results in stripes appearing on the display screen. Furthermore, since the voltage supplied to the liquid crystal rises to the predetermined voltage only after the DISP-OFF signal has turned into the active state, the screen brightens up not immediately but gradually. Therefore, according to this method, degradation of display quality is unavoidable.
Moreover, the DC--DC converter for generating the bias voltage for driving the liquid crystal is controlled in such a way that the bias voltage is generated based on the value of the power source voltage VDD irrespective of the state of the DISP-OFF signal. As a result, for example, when the power source voltage is reduced (e.g. from 5 V to 3 V) and the power source voltage takes accordingly shorter time to fall during the display-off sequence, there remains no sufficient time for the DC--DC converter to drop its output voltage before the fall of the power source voltage VDD.